The third class of postural reflex is the righting reflexes. These reflexes tend to restore an altered position of the head and body toward normal. The receptors responsible for righting reflexes include the vestibular apparatus, the neck stretch receptors, and mechanoreceptors of the body wall.
Brainstem Control of Locomotion
The spinal cord contains neural circuits that serve as central pattern generators for locomotion, as discussed earlier. These CPG circuits produce very regular rhythmic output that characterizes stereotyped behavior, such as walking. The irregularities of real-world environments, however, often require modification of this stereotyped output (e.g., if you are walking and see a hole in the floor where you are about to step, you can extend the forward swing of your leg past the hole onto solid ground beyond it).
Such modifications can be the result of sensory input to the spinal cord, as shown in
Fig. 9.10 , in which stimulation of FRA fibers in a peripheral nerve caused a phase shift in the locomotor pattern. They can also be the result of descending commands along the motor pathways discussed earlier. In this case, sensory data (e.g., visual) can be used by the brain to make anticipatory modifications in CPG activity so that potential obstacles can be avoided. In addition, people can voluntarily control activation, or shutdown, of the CPG (i.e., deciding consciously when to start and stop walking). Such voluntary regulation of spinal CPGs originates in the cerebral cortex; however, much of the cortical influence on locomotion appears to be mediated via projections to brainstem regions known as locomotor regions. A locomotor region can be defined as a brain area that, when stimulated, leads to sustained locomotion.
There are several such locomotor regions in the brain-stem, and they are located at different levels ranging from the subthalamus to the medulla and are connected with each other. The best known is the midbrain locomotor region, which is thought to organize commands to initiate locomotion. It is located in the midbrain at the level of the inferior colliculus. Voluntary activity that originates in the motor cortex can trigger locomotion by the action of corticobulbar fibers projecting to the midbrain locomotor region. The commands are relayed through the reticular formation and then to the spinal cord via the reticulospinal tracts.
Motor Control by the Cerebral Cortex
Thus far in this chapter, emphasis has been on reflexes and relatively automatic types of movement. We now discuss the neural basis for more complex, goal-directed voluntary movement. Such movement often varies when repeated and is frequently initiated as a result of cognitive processes rather than in direct response to an external stimulus. Thus it requires the participation of motor areas of the cerebral cortex.
•Fig. 9.14 Motor Areas of the Frontal Cortex. A andB, Lateralandmedialviewsofahemisphere,showingthemajorcorticalmotorareas.FEF,frontaleyefield.Theinset inB showsthewallsofthecingulatesulcus,whichcontainthecingulatemotorareas.NumbersinparenthesesareBrodmannareanumbersforthecingulatemotorareas(CMAs).C, Diagramshowinginterconnectionsofthemotorareas.PMA,premotorarea;SMA,supplementarymotorarea.